The present invention relates to light emitting diodes (LEDs) devices and fabricating LEDs. LEDs have been utilized in back light units (BLUs) for illuminating liquid crystal displays (LCDs) employed in electronic devices, such as notebook computers, cellular phones, LCD-TVs etc. For example, LED BLUs may be deployed along one or two edges of a BLU of an electronic device for illuminating the LCD without substantially increasing the thickness of the display and/or the electronic device. Given the trends of portability/mobility, miniaturization, commoditization, etc. in the electronic device industry, arrangements of LED BLUs may be required to minimize power consumption, form factors, and material/manufacturing costs of electronic devices, while maximizing light output and optimizing light beam profiles. Typically, conventional LEDs cannot satisfy these requirements.
A conventional LED may typically be supported by a sapphire- or SiC-based substrate. With the sapphire- or SiC-based substrate, the conventional LED may only be able to include a p-type ITO current spreading layer, but not an n-type current spreading layer. The p-type ITO typically has a relatively lower conductivity compared with an n-type ITO. Given the p-type ITO current spreading layer, the conventional LED may not be useful to fabricate large aspect ratio devices due to limited current spreading.
Further, the sapphire- or SiC-based substrate may typically be too fragile to have an elongated device configuration. Accordingly, the light-emitting side of the conventional LED may be limited to having a low length-to-width ratio (or aspect ratio), such as 1:1 or 2:1, in order that the conventional LED may have sufficient structural strength. When conventional LED BLUs are deployed along edges of a thin LCD, a large number of the LEDs may be required, in order to provide sufficient and homogenous illumination on the LCD. As a result, material and manufacturing costs may be increased. The large number of LEDs also may require a large amount of electricity input. As a result, power consumption may be relatively high. At the same time, a large amount of heat may be generated. The heat may degrade the performance (e.g., color) and durability of the LCD.
Conventional methods for manufacturing LEDs also may result in a high cost of the conventional LEDs and therefore may increase the cost of electronic devices. Conventional methods for making a separating semiconductor devices may include depositing layers to form numerous semiconductor devices on a wafer substrate and then utilizing mechanical techniques to separate the individual devices. The separation is typically performed by dicing or scribing the substrate to separate the individual devices. Dicing is typically done with a diamond saw, diamond scriber or laser, which may typically be a time consuming process performed by very expensive machines. Accordingly, problems associated with the conventional methods may include one or more of process yield issues, device performance issues, and processing cost issues.
1. Process Yield Issues
According to conventional mechanical device separation methods, such as dicing and scribing methods, and a laser scribing method, each individual device is separated by cutting along a grid line, or street line, between the devices with the selected method. This is a slow process since each of the street lines is cut one at a time and sequentially
Process yield issues become more significant for semiconductor devices having hard substrate materials, such as GaN on sapphire or GaN on SiC materials. Furthermore, the separation yield is greatly affected by any cracks or defects created by substrate grinding and polishing. If the cutting lines pass through defective areas, the result is very low device separation yield.
As a result, device separation is known to be the most tedious and low yield process among the entire semiconductor device fabrication processes. In practical terms, the back-end process yield for the GaN-based semiconductor fabrication is known to be as low as less than 50%, while the front-end fabrication process yield is typically in the range of above 90%.
2. Device Performance Issues
Due to the physical abrasive action of dicing and scribing, the device performance after device separation may be significantly deteriorated. For example, the LED side wall where the light emits may become damaged due to abrasive cutting action during device separation, which is the main cause of light output reduction after device separation.
In the case of laser scribing, the device separation is accomplished by melting the substrate material with a high intensity laser beam. As a result, the melted substrate material often accumulates on the side wall of the device, which results in lowering light output of the LED as well.
3. Process Cost Issue
The average die separation processing time for GaN/sapphire LED having approximately 10,000˜12,000 devices per wafer is approximately 40 min to 1 hour with the conventional separation methods. This means that one device separation machine can handle only 24 to 36 wafers per day (700˜1,000 wafers/month) if the machine operates 24 hours/day. In order to achieve a commercially desirable factory output, many machines and significant capital equipment investment is needed.
In addition, the diamond cutting wheels for dicing machine and diamond tips for the scribing machine are very expensive consumable parts, hence there are significant consumable part cost involve with the conventional die separation processes.
In the case of laser scribing, the major consumable part is the laser source. In order to maintain constant laser beam energy, the laser source gas must be recharged regularly. The laser source is the one of the most expensive components in the laser scribing system.